Prediction of micropollutant elimination during ozonation of a hospital wastewater effluent

Abstract

Determining optimal ozone doses for organic micropollutant elimination during wastewater ozonation is challenged by the presence of a large number of structurally diverse micropollutants for varying wastewater matrice compositions. A chemical kinetics approach based on ozone and hydroxyl radical ((OH)-O-center dot) rate constant and measurements of ozone and (OH)-O-center dot exposures is proposed to predict the micropollutant elimination efficiency. To further test and validate the chemical kinetics approach, the elimination efficiency of 25 micropollutants present in a hospital wastewater effluent from a pilot-scale membrane bioreactor (MBR) were determined at pH 7.0 and 8.5 in bench-scale experiments with ozone alone and ozone combined with H2O2 as a function of DOC-normalized specific ozone doses (gO(3)/gDOC). Furthermore, ozone and (OH)-O-center dot exposures, (OH)-O-center dot yields, and (OH)-O-center dot consumption rates were determined. Consistent eliminations as a function of gO(3)/gDOC were observed for micropollutants with similar ozone and (OH)-O-center dot rate constants. They could be classified into five groups having characteristic elimination patterns. By increasing the pH from 7.0 to 8.5, the elimination levels increased for the amine-containing micropollutants due to the increased apparent second-order ozone rate constants while decreased for most micropollutants due to the diminished ozone or (OH)-O-center dot exposures. Increased (OH)-O-center dot quenching by effluent organic matter and carbonate with increasing pH was responsible for the lower (OH)-O-center dot exposures. Upon H2O2 addition, the elimination levels of the micropollutants slightly increased at pH 7 (<8%) while decreased considerably at pH 8.5 (up to 31%). The elimination efficiencies of the selected micropollutants could be predicted based on their ozone and (OH)-O-center dot rate constants (predicted or taken from literature) and the determined ozone and (OH)-O-center dot exposures. Reasonable agreements between the measured and predicted elimination levels were found, demonstrating that the proposed chemical kinetics method can be used for a generalized prediction of micropollutant elimination during wastewater zonation. Out of 67 analyzed micropollutants, 56 were present in the tested hospital wastewater effluent. Two-thirds of the present micropollutants were found to be ozone-reactive and efficiently eliminated at low ozone doses (e.g., >80% for gO(3)/gDOC = 0.5). (C) 2014 Elsevier Ltd. All rights reserved.